Tumors create a pathologic state of acquired tolerance toward their own antigens. This represents a fundamental barrier to successful immunotherapy of cancer. The tryptophan-catabolizing enzyme indoleamine 2,3-dioxygenase (IDO) is an innate molecular mechanism of acquired tolerance in vivo. IDO has been implicated as an important contributing mechanism to tumor-induced tolerance, particularly during the clinically-relevant period when the tumor must re-establish tolerance to itself following chemotherapy. Despite its potential importance, however, the mechanisms by which IDO creates its potent immunoregulatory effects remain incompletely understood at the cellular and molecular level. During the previous period of support, successful studies using genetically-defined murine models have: (i) elucidated the identity of the novel subset of plasmacytoid dendritic cell (pDC) in tumor-draining lymph nodes that express IDO; (ii) identified the downstream molecular pathway in T cells by which IDO exerts key biologic effects, comprising the stress- kinase GCN2 and downstream transcription factor CHOP/gadd153; (iii) reported the novel discovery that IDO- expressing pDCs directly activate mature Foxp3+ regulatory T cells (Tregs) for potent suppressor activity via the GCN2->CHOP pathway; and (iv) shown that, when Tregs are exposed to pro-inflammatory signals from activated effector T cells, IDO and the GCN2->CHOP pathway act to maintain Tregs in their suppressive phenotype; whereas in the absence of IDO, Tregs are converted into a T-helper phenotype indistinguishable from TH17 cells. Based on these novel findings, the current proposal addresses the hypothesis that IDO functions as a key molecular switch for Tregs in tumor-draining LNs, regulating the fundamental choice between inflammation-induced activation of Treg suppressor function, versus inflammation-induced abrogation of the Treg phenotype. Aim 1 will test the hypothesis that pro-inflammatory signals from activated effector T cells drive Tregs into a forced choice - either to become potently suppressive, or to lose the Treg phenotype and convert to TH17-like cells - and that the outcome of this choice is dictated by IDO-mediated regulation of IL-6 production by pDCs and activation of the GCN2->CHOP pathway in Tregs. Aim 2 will use genetically- defined mouse models to test the hypothesis that the IDO pathway is critical in regulating the balance between Treg-mediated suppression versus pro-inflammatory TH17/effector T cell activation in tumor-bearing hosts, particularly during the critical window of recovery following chemotherapy. Aim 3 will use established B16 tumors treated with anti-tumor vaccine plus the IDO-inhibitor drug 1MT to test the hypothesis that blocking IDO allows therapeutic immunization to break tolerance to established tumors; and that the RORt-mediated conversion of Tregs to TH17-like cells is mechanistically required in order for this to occur. The long-term translational goal of these studies is to exploit the newly-available IDO-inhibitor drugs, now entering Phase I clinical trials, as a novel and clinically applicable strategy to block Treg activation by tumors.